1. Field of the Invention
The present invention relates to an optical remote airflow measurement apparatus, and more particularly to an optical remote airflow measurement apparatus that measures a wind speed in a remote region of approximately several hundred meters up to several tens of kilometers away on the basis of a Doppler Effect by emitting laser light into the atmosphere and receiving scattered light generated from the laser light in the atmosphere.
2. Description of the Related Art
In recent years, air turbulence has gained attention as a principle cause of aircraft accidents, and therefore research and development is being undertaken into a Doppler LIDAR using laser light as an apparatus installed in an aircraft to detect air turbulence in advance (see H. Inokuchi, H. Tanaka, and T. Ando, “Development of an Onboard Doppler LIDAR for Flight Safety,” Journal of Aircraft, Vol. 46, No. 4, pp. 1411-1415, July-August 2009, for example). Note that a LIDAR is a detection method employing light, and is an abbreviation for “Light Detection and Ranging”. In a Doppler LIDAR, a wind speed is measured by receiving scattered laser light generated when an emitted light beam is scattered by minute aerosols floating in the atmosphere, and measuring frequency variation (wavelength variation) therein due to the Doppler Effect. A typical Doppler LIDAR measures a wind speed in a remote region on the basis of the Doppler Effect by emitting pulse form laser light and receiving scattered laser light generated from the laser light by the aerosols in the atmosphere. A Doppler LIDAR is already being put to practical use as an apparatus disposed on the ground to measure airflows in the sky thereabove.
The present inventor previously proposed a “Wind Disturbance Prediction System” in Japanese Patent Application Publication No. 2003-14845 (Japanese Patent Publication No. 3740525) “Wind Disturbance Prediction System”, laid open on Jan. 15, 2003. An object of this invention was to provide a measurement system with which a three-dimensional wind disturbance can be measured, sudden warnings without prior notice, such as those of a conventional wind shear warning system, can be avoided, a determination as to whether or not a warning is reliable can be made in advance, wind disturbances can be detected such that countermeasures can be determined easily, few aerodynamic and structural effects are encountered when the system is installed in an aircraft, measurement can be performed at speeds of 20 to 30 m/s or less, at which measurement with a Pitot tube is impossible, measurement can be performed even when an airflow direction differs greatly from an airframe axis, and positional errors do not occur. This wind disturbance prediction system employs a method of installing a coherent laser anemometer having an inbuilt heterodyne receiver in an aircraft, emitting laser light while conically scanning the laser light, and measuring the wind speed of a remote three-dimensional airflow by receiving scattered light from a wind disturbance region in front of the airframe during flight. Further, taking into consideration the effect on the airframe of upper/lower wind and front/rear wind, information relating to the measured three-dimensional airflow is converted into upper/lower wind alone and displayed simply in two dimensions, whereby the wind disturbance is separated into and expressed as an air turbulence strength and an average wind. Furthermore, when the measured airflow information is transmitted to a pilot, a position of the disturbance is displayed using a time until the disturbance is encountered as a reference rather than a distance. In so doing, a part of a cylindrical optical system of a wind measurement LIDAR can be omitted, leading to an improvement in installation ability.
In this type of Doppler LIDAR, a technique for extracting useful signals from noise is important, and therefore, by reducing noise, measurement reliability can be improved, measurement can be performed even in a remote measurement region where a signal strength is low, and a maximum measurement distance can be increased. Typical conventional techniques involve smoothing noise by integrating measurement signals and superimposing the signals, but when a noise generation characteristic is irregular, or in other words in relation to colored noise, useful signals cannot be distinguished, and therefore the noise cannot be reduced.
Therefore, a method of measuring a unique noise pattern of a reception system in advance and subtracting the noise pattern from a measurement signal has been tested. However, this method cannot respond to variation in a condition of the colored noise during measurement, and an offset velocity cannot be varied during measurement. A measurement range of a Doppler frequency is limited, and therefore the offset velocity is a fixed velocity used in a function for subtracting a Doppler frequency corresponding to a flying speed from a measured frequency and aligning a result with a Doppler frequency range of wind speed information. The offset velocity conventionally takes a fixed value, and cannot therefore be used over a wide range of flying speeds.